During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, on a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulose fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
It should also be appreciated that the vast majority of forming, press and dryer fabrics are, or at least include as a component, a woven fabric in the form of an endless loop having a specific length, measured longitudinally therearound, and a specific width, measured transversely thereacross. Because paper machine configurations vary widely, paper machine clothing manufacturers are required to produce forming, press and dryer fabrics to the dimensions required to fit particular positions in the forming, press and dryer sections of the paper machines of their customers. Needless to say, this requirement makes it difficult to streamline the manufacturing process, as each fabric must typically be made to order.
The woven base fabrics themselves take many different forms. For example, they may be woven endless, or they may be flat woven using one or more layers of machine direction (“MD”) and cross-machine direction (“CD”) yarns, and subsequently rendered into endless form with a woven seam. Alternatively, they may be produced by a process commonly known as modified endless weaving, wherein the widthwise edges of the base fabric are provided with seaming loops using the MD yarns thereof. In this process, the MD yarns weave continuously back-and-forth between the widthwise edges of the fabric, at each edge turning back and forming a seaming loop. A base fabric produced in this fashion is placed into endless form during installation on a papermachine, and for this reason is referred to as an on-machine-seamable fabric. To place such a fabric into endless form, the two widthwise edges are brought together, the seaming loops at the two edges are interdigitated with one another, and a seaming pin or pintle is directed through the passage formed by the interdigitated seaming loops.
In any event, the woven base fabrics are in the form of endless loops, or are seamable into such forms, having a specific length, measured longitudinally therearound, and a specific width, measured transversely thereacross. Because paper machine configurations vary widely, paper machine clothing manufacturers are required to produce press fabrics, and other paper machine clothing, to the dimensions required to fit particular positions in the paper machines of their customers and therefore each fabric must typically be made to order.
Fabrics in modern papermaking machines may have a width of from 5 feet to over 33 feet, a length of from 40 feet to over 400 feet and weigh from approximately 100 pounds to over 3,000 pounds. These fabrics wear out and require replacement. Replacement of fabrics often involves taking the machine out of service, removing the worn fabric, setting up to install a fabric and installing the new fabric. While many fabrics are endless, many of those used today are on-machine-seamable. Installation of the fabric includes pulling the fabric body onto a machine and joining the fabric ends to form an endless belt.
Seams have presented significant problems in the function and use of industrial fabrics or belts in papermaking as well as nonwoven production, for example. They have a thickness, or caliper, that is different from that of the industrial fabric edges the seam is joining, and variations in caliper thickness between the seam and the fabric edges can lead to marking of the product carried on the belt. Seam failure may also result if the seam area has a greater caliper than the fabric edges as the seam becomes exposed to machine components and resulting abrasion or friction. If the belt is permeable to fluids (air and/or water), permeability/porosity differences in the seam area versus the body of the fabric can also cause objectionable marking of the products being made using the fabrics, or other operational problems.
Therefore, whether the industrial fabric is a forming, press, dryer, through-air-drying (TAD) or an engineered-fabric used to produce nonwovens by processes such as meltblowing, spunbonding or hydroentangling, or for wet processes such as a DNT belt or sludge filter belt or the like, or textile finishing belts, the properties of seam uniformity and integrity are critical.
Fabric seam terminations or the ends of the yarns that are interlaced or interwoven to form the seam are susceptible to pulling back when run on a paper, paperboard or tissue/towel or other industrial machines when the fabric is subjected to machine direction (MD) tension. To minimize this seam “pullback”, the terminal ends of the yarns in the seam are conventionally bonded to an adjacent yarn with an adhesive. However, adhesives are not fully resistant to the machine running conditions, and still allow for pullbacks or yarn slippage to occur over time. Likewise, the use of adhesives with other reinforcement means such as sewing terminal ends of a paper machine clothing (PMC), TAD or engineered fabric does not produce the desired seam integrity or uniformity either.
In addition, the width of the seam area, as measured in the MD, formed using conventional techniques typically range, for example, anywhere between three and one half to twenty inches or even more. Therefore, for many reasons, it is desirable to reduce the MD length of this seam area.
FIGS. 1(a-d) show the problems associated with conventional seam formation techniques for a TAD fabric, for example, wherein the terminating ends of the two fabric edges are rewoven into the fabric, “overlapped” in the seam area and the critical points 512, where these ends might “pullback” in the MD and the ends themselves might protrude through the paper side surface, are identified (FIG. 1a). Eventually, the slippage in the overlapping area increases as shown by the arrows due to increased localized stresses in the fabric (FIG. 1b) and there is a complete slippage and a hole 516 appears in the seam area of the fabric (FIG. 1c). Accordingly the overlap region of the seam is typically reinforced by manually gluing 518 (FIG. 1d) to increase its strength; however, gluing is a laborious and time consuming process. Due to its low precision, it is also hard to limit the glue to only the overlapping yarns. In addition, the glue eventually either fails due to flexing of the fabric and/or abrasion as the fabric is run on the paper machine.
Accordingly, there is a need for a different or improved means of strengthening seam yarn end terminations, and consequent seam strength.
One possible technique for strengthening seam yarn end terminations for fabrics is thermal welding, such as ultrasonic welding. Ultrasonic refers to sounds that are above the range of human hearing, i.e. >20,000 Hz, and ultrasonic welding refers to the fusing of materials using sound waves. Many attempts have been made to use ultrasonic energy to join fabrics edges together, i.e. to join lengths of fabric into endless forms to produce an endless belt.
However, unacceptable seam formation results often arise from ultrasonic welding such as excessive melting of the yarns, reduced seam permeability, and distortions in the fabric due to localized yarn shrinkage, all stemming in part from the fact that conventional ultrasonic welding is based on modifying multiple parameters of time, energy and distance.